1. Field of the Invention
The present invention relates to a semiconductor device in which an active region for a thin-film transistor (TFT) is formed on an insulating substrate or on an insulating film formed on a substrate; and a method for producing such a device. More particularly, the present invention relates to a semiconductor device which is useful to an active-matrix liquid crystal display device; and a method for producing such a device.
2. Description of the Related Art
It is known to use as a semiconductor device including TFTs on an insulating substrate made of glass, etc., an active-matrix liquid crystal display device using the TFTs for driving the pixels, an image sensor and the like. In general, a thin-film silicon semiconductor is used as a material for the active regions of the TFTs provided for such a device. Such a thin-film silicon semiconductor is roughly classified into two categories; namely, that made of an amorphous silicon (a-Si) semiconductor and that made of a crystalline silicon semiconductor.
Of the above-mentioned two types of thin-film silicon semiconductors, an amorphous silicon semiconductor is currently used most frequently for general applications. This is because an amorphous silicon semiconductor may be mass-produced using a vapor-phase growing method more easily and at a relatively low temperature as compared with a crystalline silicon semiconductor. Despite these advantages, the physical properties, e.g., conductivity, of the amorphous silicon semiconductor are inferior to those of the crystalline silicon semiconductor. Thus, in order to realize higher performance characteristics, there has been a great demand for the establishment of a method for producing a TFT having a crystalline silicon semiconductor. Examples of a crystalline silicon semiconductor include polycrystalline silicon, micro-crystalline silicon, amorphous silicon containing a crystalline component, and semi-amorphous silicon exhibiting an intermediate state between crystallinity and non-crystallinity.
The following three methods are currently employed for obtaining the above-mentioned thin-film silicon semiconductor exhibiting some crystallinity.
(1) A crystalline silicon semiconductor film is grown directly on a substrate during the deposition of the film. PA1 (2) An amorphous silicon film is initially deposited, and subsequently crystallized using laser beam energy or the like. PA1 (3) An amorphous silicon film is initially deposited, and subsequently crystallized by the application of thermal energy thereto.
These conventional methods, however, have the following problems.
In the case of employing method (1), the deposition and the crystallization of the film proceed simultaneously. Therefore, it is indispensable to deposit a thick silicon film in order to obtain a crystalline silicon composed of grains having a large size. However, it is technologically difficult to uniformly deposit a film having satisfactory semiconductor physical properties over the entire surface of a substrate. Furthermore, since such a film is deposited at a relatively high temperature of 600.degree. C. or more, an inexpensive glass plate is unsuitable for a substrate which may be used in this method, so that the necessary cost becomes disadvantageously high.
In the case of employing method (2), a crystallization phenomenon is utilized during a process for melting and solidifying a film. As a result, the grain boundaries are satisfactorily treated even though the grain size of the resulting crystal is small. Thus, a crystalline silicon film of high quality may be obtained. In spite of these advantages, in the case of irradiating an excimer laser beam which is currently used most frequently, the area to be irradiated with a laser beam is small, so that throughput is disadvantageously low. In addition, the stability of the excimer laser is not sufficient in order to uniformly treat the entire surface of a large-scale substrate. In light of these problems, method (2) cannot help being regarded as a next-generation technology.
It is true that method (3) has an advantage of being applicable to the treatment of a larger-scale substrate as compared with methods (1) and (2), however according to method (3), a heat treatment is required to be conducted at a high temperature of 600.degree. C. or more over several tens of hours so as to realize the crystallization. Thus, in order to reduce costs by using an inexpensive glass substrate and improve the resulting throughput, two incompatible purposes must be fulfilled at the same time: i.e., the heating temperature should be lowered for the purpose of reducing the cost; and the crystallization should be realized in a short period of time in order to improve the throughput. In addition, since method (3) utilizes a solid phase crystallization (epitaxy) phenomenon, crystal grains are laterally grown in parallel with respect to the substrate surface, so that grains having a size of several .mu.m are obtained. As a result, the crystal grains thus grown come into contact with each other so as to form grain boundaries. Since these grain boundaries function as a trap level for carriers, the presence of the grain boundaries is very likely to cause the decrease in field-effect mobility of TFTs.
In view of solving the above-mentioned conventional problems, Japanese Patent Application No. 5-218156 discloses a method for producing a crystalline silicon thin film in order to fulfill at the same time the two purposes required for the crystallization, i.e., lowering the annealing temperature and the shortening the necessary process time; and to suppress the effects of the grain boundaries to a minimal level.
According to the method of the above-identified patent application, a very small amount (e.g., on the order of 1.times.10.sup.18 cm.sup.-1) of impurity elements such as nickel, palladium and zinc is introduced into an amorphous silicon film as a nucleus for crystal growth, thereby accelerating the nucleus generation rate at an initial stage of the crystallization and the nucleus growth rate in the subsequent stages. As a result, satisfactory crystallinity can be obtained at a low temperature of 580.degree. C. or less for a short period of time of about four hours. The probable mechanism of this growth may be understood as follows: first, a crystal nucleus is generated by introducing impurity elements as a nucleus in an earlier stage; then, the impurity elements promote crystal growth as catalysts, thereby accelerating crystallization. Hereinafter, the impurity elements of this type will be referred to as "catalyst elements" in the above-described sense.
According to this method, aside from enabling crystallization with a laser beam, a crystalline silicon film and an amorphous silicon film may be selectively formed on one and the same substrate by introducing the catalyst elements into a portion of the substrate. On the other hand, if the heating process (annealing process) is further continued after crystallization, the portion in which the crystal has been growing (hereinafter, this portion will be referred to as a "crystal-growing portion") expands in a lateral direction (or a direction parallel to the surface of the substrate) from the crystallized portion by the selective introduction of the catalyst elements towards the amorphous portion surrounding the crystallized portion. Hereinafter, the crystal-growing portion in a lateral direction will be referred to as a "laterally growing portion". In this laterally growing portion, a plurality of needle-like or column-like crystals extend along the crystal-growing direction parallel to the substrate, and grain boundaries do not exist in the growing direction. Accordingly, if a channel potion for a TFT is formed using this laterally growing portion, then a high-performance TFT may be realized.
Referring to FIG. 14, a process for producing a TFT using this laterally growing portion will be described below. FIG. 14 is a plan view showing a TFT seen from above the upper surface of the substrate.
First, a mask constituted by an insulating film made of silicon dioxide or the like is deposited over the amorphous silicon film formed over the entire surface of the substrate. Then an opening 500 for adding catalyst elements is formed through the mask, and the catalyst elements are introduced through the opening 500 into the amorphous silicon film.
Next, a heat treatment (annealing) is conducted at approximately 550.degree. C. for about four hours. As a result, the amorphous silicon film region under the opening 500 to which the catalyst elements have been added (catalyst element added region) is crystallized, while the other regions remain amorphous silicon. Then, the heat treatment is further continued for about eight hours, so that the crystal goes on laterally growing in the growing direction 501 extending from the catalyst element added region, thereby forming the laterally growing portion 502.
Subsequently, a TFT is formed according to a conventional method by using this laterally growing portion 502. In this case, if a source region 503, a channel region 504 and a drain region 505 are provided at the positions with respect to the laterally growing portion 502 as shown in FIG. 14, the moving direction of the carriers accords with the crystal growing direction 501. Consequently, a high mobility TFT in which grain boundaries do not exist in the moving direction of the carriers is realized.
In a TFT thus produced, the mobility of an N-channel type TFT is in the range of 80 to 100 cm.sup.2 /Vs, while the mobility of a P-channel type TFT is in the range of 60 to 80 cm.sup.2 /Vs. If such a TFT is used for a liquid crystal display device, then, in addition to the display portion, i.e., the switching elements in an active-matrix region, peripheral driving circuits such as an X decoder/driver and a Y decoder/driver may be fabricated on one and the same substrate during a single production step.
FIG. 15 is a block diagram showing an electrooptic system of a liquid crystal display device including a display, a CPU, memories and the like. In FIG. 15, the region surrounded by the one-dot chain is the region for producing the display portion on one and the same substrate made of glass, etc. by using the technique disclosed in above-mentioned Japanese Patent Application 5-218156. However, in order to fabricate a product at an even lower cost; to down-scale the module; and to simplify the mounting process, it is necessary to realize the integration at a higher level. Therefore, it is preferable to construct the entire electrooptic system on an identical substrate as shown in FIG. 15.
However, it is not sufficient to construct the entire system on an identical substrate. This is because the semiconductor device constituting the CPU is required to operate at an even higher speed as compared with the semiconductor device constituting the peripheral driving circuits. Therefore, according to the technique disclosed in Japanese Patent Application 5-218156, the mobility in the TFT is not satisfactory. Thus, the CPU cannot be formed on an active-matrix substrate on which an active-matrix region is formed. This explains why an IC chip formed using a single crystalline silicon substrate is conventionally mounted on an active-matrix substrate.
Accordingly, if a crystalline silicon film having a mobility substantially equal to a mobility of a single crystalline silicon could be formed on a transparent insulating substrate made of glass, etc., then not only the performance of the peripheral driving circuits for driving an active-matrix region might be remarkably improved, but also a liquid crystal display device including a display, a CPU, memories and the like might be formed, and in addition, the liquid crystal display device might function as an image sensor, a touch operator, and the like.